Thermodynamic Properties of Oxygen Molecules at High Temperatures

Calculations of the second virial coefficients and their derivatives, obtained by fitting the Hulburt-Hirschfelder potential to accurate ab initio quantum mechanical calculations for the seven bound states of molecular oxygen that dissociate to ground state atoms, are used to determine thermodynamic properties of oxygen molecules at high temperatures. Unlike the usual statistical thermodynamic method for obtaining partition functions by summing over vibrational-rotational energy levels, the virial coefficient method depends on integrating over the potential energy. This may provide an accurate description of energies near the top of the potential energy well, where vibrational-rotational energy levels are usually not accurately determined. This makes the virial coefficient method particularly useful for predicting high-temperature thermodynamic properties outside the range of laboratory investigation and beyond the limits of the partition function approach. In the work presented here, the virial coefficient method is used to calculate the heat capacity and enthalpy of oxygen molecules up to 25,000 K. © 1996 Plenum Publishing Corporation.